This invention relates to an electric motor unit, and more specifically, to an electric motor unit for rotating a polygonal mirror.
In general, a polygonal mirror-type optical deflector is provided with a motor unit to rotate a polygonal mirror at high speed, e.g., scores of thousands of revolutions per minute. In such a motor unit, friction between a motor shaft and bearings should be minimized for high-speed rotation. To meet this requirement, Japanese Patent Publication No. 6854/78 teaches a motor unit of a tilting pad type. In the motor unit of the tilting pad type, the motor shaft is radially supported by journal bearings of a dynamic pressure type, and is suspended so as to be axially supported by a repulsive force produced between permanent magnets fixed individually to an end of the motor shaft and a motor housing. Having its motor shaft supported both radially and axially, this motor unit is suitable for high-speed rotation, though it has the following drawbacks. Suspended by the repulsive force between the pair of permanent magnets, the motor shaft is liable to vibrate due to external vibration or the like, as well as to become somewhat eccentric. Moreover, the arrangement of the permanent magnets along the axis of the motor shaft may lead to an increase in size of the motor unit.
The use of the motor unit involving these problems in an optical deflector of the polygonal mirror type will cause the following additional problems. Since the incidence position of a laser beam incident upon the polygonal mirror changes as the motor shaft vibrates along its axis, the width of the polygonal mirror must be sufficient. Therefore, the polygonal mirror increases in cost and weight, so that the rotatory load on the motor unit increases, and thereby reduces the starting capability of the motor unit. In a deflector designed so that the light reflecting surfaces of the polygonal mirror are at an angle to the axis of the motor shaft, the scanning rate of a laser deflected by the polygonal mirror varies as the incidence position of the laser beam is changed by the vibration of the motor shaft.
U.S. patent application Ser. No. 411,959 filed on Aug. 26, 1982 now U.S. Pat. No. 4,443,043 issued April 17, 1984, discloses a motor unit intended to solve the above-noted problems inherent in the conventional motor unit. In this U.S. Application, the motor shaft is supported by a pair of radial bearings of dynamic pressure type and by the attractive force between pairs of ring magnets. According to the magnetic thrust bearing employed in the motor unit, the motor shaft is suspended by the attractive force. Thus, the motor shaft is unlikely to vibrate even if external force is applied thereto. In addition, since the shaft and the ring magnets are coaxially arranged, enlargement of the motor unit can be prevented. However, the motor unit proposed in the U.S. Application necessitates a high assembly precision, making it necessary to allow sufficient time for processing, assembly and adjustment. Naturally, the motor unit in question is low in productivity, making it difficult to keep the manufacturing cost down. To be more specific, one of the paired radial bearings of the dynamic pressure type is fixed to the motor housing, with the other radial bearing being fixed to a cover detachably fixed to the motor housing, in order to facilitate the assembly and disassembly of the motor unit. Thus, in the assembly of the motor unit, the motor shaft and the paired radial bearings must be aligned coaxial within an error of, for example, about 5 .mu.m. It follows that it is necessary to process the cover, housing, bearings and shaft with a sufficiently high accuracy. In addition, it is necessary to allow sufficient time for the alignment, leading to low productivity and high manufacturing cost.
The U.S. Application also discloses dynamic pressure type radial bearings comprising herringbone-shaped grooves. In general, herringbone grooves are formed to a depth of 3 to 6 .mu.m on the shaft surface by photoetching, rolling or cutting. However, the arrangement of the grooves themselves is relatively complex, requiring a long processing time for the motor shaft. Naturally, the motor shaft manufacturing cost increases. Also, in the dynamic pressure type radial bearings comprising herringbone grooves, air is introduced through the grooves by utilizing the viscosity resistance of the air layer formed between the rotating shaft and the bearing, with the result that the rotating direction of the bearing is determined by the arrangement, i.e., inclination, of the grooves. It follows that it is necessary to determine in advance the rotating direction of the motor shaft. When it is desired to change the rotating direction from, for example, the clockwise direction to the counterclockwise direction, it is necessary to use another motor unit.